Fiber tensile strength model

Based on the same average fiber tensile strength model as that employed in Section 4.2.3, the fiber fragmentation criterion is derived in terms of the external stress, ffa(= (h = o er, for the partially debonded interface ... 

Fig. 4.12. Varialion of mean fiber fragment length, 2L, versus applied strain, t, in the full bonded interface model (---------) varying ctts(22,) (------------) constant fiber tensile strength ctts- After Kim et al. (1993b).

The mean fiber fragment length, 2L, is plotted as a function of the applied strain, e, in Fig. 4.18. Similar to the results for the fully bonded interface model shown in Fig. 4.12, the full frictional interface model predicts that (2L) decreases sharply with increasing applied strain within a short range of c. A high fiber tensile strength... 

Fig. 4.18. Variation of mean fiber fragment length, 2L. as a function of applied strain, e, predicted in the fully debonded interface model for constant fiber tensile strengths

Apparently this model in its present form does not predict a fibrillated fracture morphology caused by shear failure, as has been clearly observed for the PpPTA, PBO and PBT fibers. Knoff noticed the similarity between the tensile failure morphology of PpPTA fibers and that of a uniaxially oriented fiber-reinforced composite. The latter fails in tension via matrix shear failure initiated at the fiber ends. This made him to conclude that, if the shear forces at a discontinuity exceed the shear strength of the bond between the fibrils, the fiber tensile strength should be proportional to the fiber shear strength [182]. 

Thermotropic polyesters are melt-spun from the nematic phase and orient easily in an elongational flow field (moderate drawdowns/forces are sufficient). In the fiber case, highly oriented fibers form easily with an initial modulus close to theory—typical values range from about 70 to 150 GPa. Ward [46] has shown that the tensile modulus may be described by an aggregate model, i.e., the modulus is a function of the inherent chain modulus, the molecular chain orientation, and the shear modulus (which described the stress transfer between chains). The tensile strength of LCP fibers follows the prediction of the lag-shear model [47]. Both the aggregate model and the lag shear model treat the LCP as though it... 

As the proposed modeling scheme is based on the kinetic processes of the resin, tensile strength at lower temperatures, where fiber failure occurred, cannot be... 

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